One engine speed same as both engines

[jamesyboyz]

Hi all,

is anyone able to explain why if I run one engine I get 7 kts at a particular rpm then if I run both I only get 8 kts with both at the same revs? This does not change with increased revs.
I just wonder why I just don’t cruise with one engine in idle and the other running normally.

 

[Deleted User YDKXO]

Its about max hull displacement speed. It takes very little power to move a boat through the water at very slow speed but the faster the boat goes the disproportionately more power the boat requires to achieve those faster speeds. And then when the hull gets close to its max hull displacement speed (1.34 x sq root of waterline length) it takes an even more disproportionate amount of power to get past that speed which is why planing boats have powerful engines and fuel consumption shoots up as the speed goes over the max hull displacement speed

My guess is that 8kts in your boat is getting close to the max hull displacement speed (what length is it?). As for cruising on one engine, yes you may be able to do that but the accepted wisdom is that you could damage the gearbox attached to the non rotating prop because that prop is trying to rotate as water passes through it. Might be worth checking with the gearbox manufacturer. Some boats are fitted with clamps allowing the non rotating shaft to be clamped to avoid loading the gearbox

 

[Cathy*]

Thank you both for the question and the first answer. We've found the same thing and normally potter around using one engine at about 7 or 8 kn. I've now checked the manual for our ZF gearbox and we're ok doing what were doing.

 

[5teve]

Could it also be that given the props pitch that a given rpm gives a certain speed regardless of number of engines.. in a perfect no slippage prop world...2 engines give more power to get on the plane as Bruce mentioned above. The additional knot is probably the result of not dragging the non operational prop.

It's like expecting a 2 engined car to go faster at a certain rpm.. the wheels and gearbox determine the speed regardless of power...

Steve

 

[QBhoy]

what boat is this happening on ? What engines ? The max displacement speed theory is most likely, but I’m not sure I’ve known such an extreme example, thinking about it.

 

[MapisM]

is anyone able to explain why if I run one engine I get 7 kts at a particular rpm then if I run both I only get 8 kts with both at the same revs?

Because with just one propeller, you have to fight both the drag of the other one (as 5teve mentioned), and also the drag introduced by the asymmetrycal thrust, which by definition requires some constant rudder correction.
This translates in a propeller effort (AKA "propeller demand") higher compared to what the same prop at the same rpm must deal with when running in parallel with the other one.
And a higher propeller demand means a higher slip (i.e. lower speed), and also higher engine load and fuel burn - as you could easily check on the engine monitoring displays, if electronically controlled.

In extreme cases, albeit very rare, you could even have a higher fuel burn while running with one engine at 7kts vs. two engines at 8kts.
And mind, I'm talking of TOTAL fuel burn.
The fact that each engine, when running alone at 7 kts, will burn more than when running together with the other one at 8kts, that is a given.
It's just a matter of how much more.

 

[Irish Rover]

My 34 foot power cat has 2 x 75HP Yanmars. Apart from manoeuvring in the marina I only ever use one engine. I generally operate it at 2200rpm and it makes 7 knots in most sea conditions. My average fuel consumption is 3.5 lph over 2 seasons and a combined 760 hours operation (380 x 2). If I operate both engines together at 2200rpm I make just over 8 knots in line with the OP's experience.

 

[jamesyboyz]

what boat is this happening on ? What engines ? The max displacement speed theory is most likely, but I’m not sure I’ve known such an extreme example, thinking about it.

Princess 30 with 28ft waterline, 2x60hp engines with 1:93 gearbox ratio.

 

[QBhoy]

Strange. Is it a 30ds perhaps ?

 

[jamesyboyz]

Strange. Is it a 30ds perhaps ?

30 DS that’s right.

 

[QBhoy]

30 DS that’s right.

ok. Lovely boats. Do they not have a planing hull ? If so, it would throw the max displacement speed theory out the window maybe ? Perhaps not, if it’s been engined with lower power. Not sure I’ve seen the 30ds hull out the water. Perhaps it’s semi displacement. Not sure. Still think it’s an extreme example, all being well with props and engines